"Heat tracing" systems for maintaining a liquid carried in a pipeline at an elevated temperature are well known in the art. Early systems employed electric resistance heaters running the length of the pipeline to provide the necessary heat or, alternately, employed conduits attached to the exterior of the pipeline through which steam or a high temperature liquid was conveyed. Many of these systems are energy intensive and prohibitively expensive to operate at current energy costs.
More recently, a system utilizing the skin effect of alternating current for generating heat has been suggested. A method and apparatus for heating a pipeline utilizing this principle was disclosed in U.S. Pat. No. 3,293,407, issued to Ando, which is hereby incorporated by reference. According to this patent, the basic elements of the skin effect current heating system are a ferromagnetic tube and an insulated conductor disposed within the tube. In a simple embodiment of this system, one end of the conductor is connected to one end of the ferromagnetic tube and the opposite ends of the conductor and tube are connected to a suitable AC source. According to the principle of operation, the electromagnetic fields generated between the tube and the conductor, cause the current in the tube to be concentrated at its inner surface. This current concentration at the inner surface of the pipe generates heat. By suitably joining the heating tube to a pipeline, the heat generated can be transmitted to the liquid transported through the pipeline. It has been found that this type of pipeline heating system is very economical and reliable in many applications as compared to the electric resistance and steam based heat tracing systems.
In many, if not most applications to date for skin effect heating systems, the power cable disposed within the heating tube operated at a temperature less than 150.degree. C. and at an applied voltage of less than 2 KV. For these moderate heating applications, commercially available power cable could serve as the insulated conductor without sacrificing performance and/or cable longevity. Recently, however, it has been found desirable to be able to provide a skin effect heating system for a comparatively long pipeline and to maintain the material conveyed in the pipeline at a temperature in excess of 120.degree. C. In one particular heating application, it was necessary to provide a skin effect heating system for a 20 mile long pipeline carrying liquified sulfur. To provide the requisite heating for this particular application, the power cable must operate at a continuous temperature of 210.degree. C., with an applied voltage of up to 5 KV RMS.
Although power cables, capable of operation at 5 KV when used in more conventional applications, were available from cable manufacturers, it was found that these commercially supplied cables could not meet the required parameters when placed inside a heating tube operating at 210.degree. C. It must be remembered that in conventional applications for power cable, the cable is strung in open air or alternately placed in a conduit or raceway. In the latter case, the conduit serves only as a protective device and not as a current carrying conductor as does the heat tube in a skin effect heat system. When commercially available cables were confined within the heating tube, the added voltage stresses and high temperature would combine to cause the premature failure of the cable. It is believed that the premature failures were due in part to the presence of insulation damaging corona.
In designing power cables to meet specified operating parameters, cable manufacturers tend to apply well known and in some cases simplified equations to arrive at the required insulation thickness, such as EQU S=V/T
where S is the mean stress across the insulation; V is the applied voltage and, T is the insulation thickness. In general, the maximum stress S that an insulation can withstand is known and therefore the required thickness T of the insulation can be easily calculated.
For calculating the required insulation thickness for an electrical system utilizing concentric conductors, i.e. a shielded cable that comprises a central conductor surrounded by and insulated from, an outer conductor, cable designers generally apply the stress equation ##EQU1## where S is the voltage stress at a radius r from the center of the inner conductor, usually expressed in volts/mil; K is the dielectric constant for the insulating material; V is the applied voltage; r.sub.oc is the radius of the outer conductor; and, r.sub.ic is the radius of the inner conductor.
For an electric system having true "concentricity" between the conductors, the stress equation yields an adequate insulation thickness. In a typical skin effect heating system, however, the insulated power cable normally lies on the bottom of the heat tube and is therefore, in reality, eccentrically positioned with respect to the outer conductor (the heat tube). The above described stress equation does not address this eccentricity and consequently yields an inadequate amount of insulation. The stresses placed on the cable especially in high temperature applications often resulted in premature failure. Thus it has been found that commercially available power cable, designed using conventional methods and equations, was unacceptable for high temperature, high voltage skin effect heating systems.
In fabricating a skin effect heating system, the heat tube is generally fastened or welded, to the pipeline as it is assembled. A power cable is then pulled through the heating tube and suitably attached to terminals and/or power sources. For very long pipelines, the heat tube is formed in sections and a length of power cable is pulled through each section and then spliced or joined to an adjacent length of cable.
The splicing of adjacent lengths of power cable is very critical, for not only must the junction withstand the high operating temperature and voltage, it is equally important to insure that the conductor junction is not in itself a source of impedance which could produce a "hot spot" at the splice. Canadian Pat. No. 1,021,836 to Ando illustrates a power cable construction as well as a method for joining two lengths of the power cable. Although the illustrated cable and splice perform satisfactorily at moderate operating temperatures and voltages, it is believed that they could not withstand both an applied voltage of 5 KV and a continuous operating temperature of 210.degree. C.